The study led by Christopher Drake, PhD, investigator at the Henry Ford Sleep Disorders and Research Center, showed that about 2-3 cups of coffee taken at bedtime, three and even six hours prior to bedtime significantly disrupts sleep.
Even when caffeine was consumed six hours before going to bed, objectively measured total sleep time was dramatically reduced (more than one hour).
Drake said that drinking a big cup of coffee on the way home from work can lead to negative effects on sleep just as if someone were to consume caffeine closer to bedtime. People tend to be less likely to detect the disruptive effects of caffeine on sleep when taken in the afternoon.
Drake and his research team studied 12 healthy normal sleepers, as determined by a physical examination and clinical interview. Participants were instructed to maintain their normal sleep schedules.
They were given three pills a day for four days, taking one pill at six, three and zero hours prior to scheduled bedtime. One of the pills contained 400 mg of caffeine, and the other two were a placebo. On one of the four days, all three pills were a placebo. Sleep disturbance was measured subjectively with a standard sleep diary and objectively using an in-home sleep monitor.
The results suggest that caffeine generally should be avoided after 5 p.m. in order to allow healthy sleep.
The study was published in Journal of Clinical Sleep Medicine.